1. Field of the Invention
This invention relates to zoom lenses and, more particularly, to zoom lenses of increased relative aperture with good optical performance over the entire zooming range, while still permitting the physical size to me minimized with the total length shortened to be suited to photographic cameras, video cameras, etc.
2. Description of the Related Art
To photographic cameras and video cameras there has been a demand for a zoom lens having a good compromise between the requirement of increasing the relative aperture with a high range and the requirement of reducing the bulk and size in such a manner that high grade optical performance is preserved.
Of these, for the video camera, because of its image pickup element being relatively low in sensitivity, the zoom lens has to get as high a relative aperture as possible.
At present, the 2/3 in. image pickup tube is widely used in the video camera from the two points of view of compactness and image quality. Also, from the standpoint of good manageability and high facility for further minimization of the size, 8 m/m video cameras are coming to be used in gradually increasing numbers. The image pickup tube to be used in this camera is required to be furthermore reduced in size while preserving high grade imagery. Recently the 1/2 in. image pickup tube or plate has found its use in 8 m/m video cameras.
If the zoom lens is of the so-called 4-unit type in principle, it is in general case that an increase of the relative aperture to as high as 1.4-1.6 or thereabout in F-number can be achieved when proper rules of lens design are set forth particularly for the fourth lens unit that is arranged on the image side of the zoom section to be stationary during zooming, and the image forming section or the fifth lens unit to well correct the residual aberrations of the zoom section.
Also, in order to shorten the total length of the entire lens system, the effective method is to reduce the bulk and size of the front or first lens unit. To allow for this to be achieved, the F-number may be increased. But, to avoid the F-number from being so much increased, it becomes important to set forth proper rules of design for the image forming section.
In addition thereto, if the reduction of the physical size of the entire lens system and the increase of the relative aperture are attempted by relying merely on strengthening of the refractive power of each lens unit, then the spherical aberration in the paraxial region, the coma from the zonal to the marginal region, and higher order aberrations such as sagittal halo are increased largely. So, it becomes difficult to get high optical performance.
Suppose, for example, the front or first lens unit is selected for shortening the total length by the method of increasing the refractive power, then the overall magnifying power of the zoom section up to the image forming section has to be increased. As a result, the first lens unit produces many aberrations, and the tolerances for the lens design parameters becomes severer. Thus, it becomes difficult to assure the prescribed optical performance.
Also, in the case of using the 2/3 in. image pickup element, according to the prior art, the total length L of the entire lens system in terms of the diagonal .phi..sub.A of the effective picture frame falls in a range of 10.phi..sub.A to 12.phi..sub.A, as disclosed in Japanese Laid-Open Patent Application No. Sho 60-260912. This implies that the total length of the entire lens system is caused to become comparatively long. Like this, it has been very difficult to achieve a reduction of the total length L to shorter than 10.phi..sub.A in such a manner that good optical performance is preserved throughout the zooming range. As other concomitant techniques mention may be made of U.S. Pat. Nos. 4,518,228, 4,525,036, 4,618,219, 4,621,905, 4,653,874, and 4,659,187.